The inner surface of a reactor vessel lid is in primary cooling water at the operation of a reactor. Therefore, it is requested that WJP be executed as maintenance to reduce primary water stress corrosion cracking at the weld portions of the inner surface of the reactor vessel lid. The WJP is executed in a state in which an underwater environment is formed on the inner surface of the reactor vessel lid 112. On the other hand, since a control rod drive unit is installed over the reactor vessel lid, it is necessary to prevent the control rod drive unit from being submerged in water at the execution of the WJP.
Note that techniques described in Patent Literatures 1 and 2 have been known as conventional WJP execution methods.